The present invention relates a control device responsive to control signals for electrically coupling an input node to an output node. More specifically, the present invention relates to a control device that makes use of a controlled impedance for varying impedance values continuously over a range of impedance values based on sensed parameters to maintain a selected output impedance value.
Integrated circuit devices frequently make use of an output driver circuit for providing output data to an output terminal. A conventional output driver circuit is described in U.S. Pat. No. 6,489,837, which includes an input terminal, an output terminal, P-channel transistors, n-channel transistors, and an input/output (I/O) pad. When a logic low signal is applied to the input terminal, the p-channel transistors are all turned on, thereby coupling the I/O pad to the positive supply voltage terminal. In this condition, the driver circuit presents a resistance to the I/O pad. This resistance is determined by the on resistances of the p-channel transistors taken in parallel.
Similarly, when a logic high signal is applied to the input terminal, n-channel transistors are all turned on, thereby coupling the I/O pad to the ground terminal. In this condition, the driver circuit presents a resistance to the I/O pad. This resistance is determined by the on resistance of the transistors taken in parallel.
It is sometimes desirable that the resistances presented to the I/O pad have a predetermined relationship with an external resistance, which is coupled to the I/O pad. For example, it may be desirable for the resistance presented to the I/O pad to match an impedance of a trace or wire coupled to the I/O pad to improve signal integrity.
Another example where it is desirable to have the resistance presented to the I/O pad have a predetermined relationship with an external resistance which is in the testing of the semiconductor integrated circuit devices. A driver within a tester is often used to apply a voltage signal at a selected high or low level to an input pin of a device under test (DUT) in order to place the DUT in a desired state. It is sometimes desirable to use a transmission line, such as a coaxial cable or micro strip structure, for signal propagation between an output pin of the tester and an input pin of the DUT. In this case, it is desirable to backmatch the output resistance of the driver with the characteristic impedance of the transmission line in order to avoid reflections in the signal path between the driver and the input pin of the DUT.
There are numerous problems associated with accurately providing a predetermined relationship between the output resistance of the driver and the external resistance coupled to the output pin or I/O pad. One problem is that the output resistance of the driver has an initial tolerance resulting from process parameter variation. This initial tolerance alters the relationship between the output resistance of the driver and the external resistance coupled to the output pin or I/O pad.
Another problem is that the output resistance of the driver tends to vary with environmental conditions such as temperature, drive voltage and aging effects, to name a few. Change in the output resistance results in a change in the relationship between the output resistance and the external resistance. Deviations from this predetermined relationship results in voltage variation at the external resistance connected to the I/O pad.
Yet another problem associated with providing a driver having an output resistance that has a predetermined relationship with the external resistance is that the external resistance coupled to the I/O pad may have an initial tolerance. For example, in the case where the external resistance coupled to the I/O pad is a transmission line, it may be difficult to maintain constant 50-ohm characteristic impedance in practical transmission line structures. Variations of plus or minus 3 ohms are quite common. A 3-ohm variation, which is a 6 percent variation in a transmission line impedance, results in a corresponding 6 percent initial amplitude error in the pulse delivered to the input pin of the DUT. Furthermore, reflections in the signal path between the driver and the input pin of the DUT can further vary the initial amplitude delivered to the input pin of the DUT. Therefore, deviations in resistance of either the output resistance or external resistance result in voltage variation at the I/O pad. In applications where timing critical these variations should be minimized.
Various solutions have been proposed to address one or more of the above problems such as that disclosed in U.S. Pat. No. 6,489,837. In this solution, a digital control circuit is used to select pull-up or pull down transistors to make discrete changes to the output resistance to better approximate the desired output resistance. These previously used solutions have shortcomings, however. It would be therefore desirable to have a circuit suitable for use as an output driver circuit which addresses the above discussed problems to create a highly accurate relationship between the resistances presented to the I/O to the external resistance coupled to the I/O pad.